The present invention is directed to a chopping energization control device which is associated with each of a plurality of phase coils which are provided on a stator of an electric motor such as a switched reluctance motor.
In switched reluctance motors, a chopping energization control is made by using an H-type switching circuit which is disclosed in Japanese Patent Laid-open Print No. Hei. 7-274569 published without examination in 1995. This switching circuit, as shown in FIG. 8, includes a first switching element 5, a second switching element 6, a first diode 7, and, a second diode 8. The first switching element 5 is interposed between one end of a phase coil 2 and a higher voltage line 3 of a power supply (not shown), while the second switching element 6 is interposed between the other end of the phase coil 2 and a lower voltage line 4 of the power supply. The first diode 7 is interposed between one end of the phase coil 2 and the lower voltage line 4 in such manner of allowing only one-way current pass from the latter to the former, while the second diode 8 is interposed between the other end of the phase coil 2 and the higher voltage line 3 in such a manner of allowing the current to pass only from the former to the latter. As each of the first switching element 5 and the second switching element 6, an insulated gate bipolar transistor (IGBT) is employed.
In the foregoing circuit, while both the first switching element 5 and the second switching element 6 are being turned on a DC current passes through the phase coil 2. On the other hand, when both the first switching element 5 and the second switching element 6 are being turned off or when one of the first switching element 5 and the second switching element 6 is turned on and the other is being turned off, the DC current fails to pass through the phase coil 2. The chopping energization control is initiated depending on a difference between the actual current value which passes through the phase coil 2 and a target current value which is to be passed therethrough and approximates the actual current value to the target current value.
The chopping energization control has three switching modes: a first mode, a second mode, and a third mode. The first mode is to repeat cyclically a condition under which both switching elements 5 and 6 are being concurrently turned on as shown in FIG. 8(a) and a condition wherein the both switching elements 5 and 6 are being concurrently turned off as shown in FIG. 8(b). The concurrent turning-on both the switching elements 5 and 6 as shown in FIG. 8(a) allows the current to pass through the phase coil 2, while concurrent turning-off both the switching elements 5 and 6 as shown in FIG. 8(b) causes a feedback current to the power supply due to an induced voltage in the phase coil. Such cyclic repetition of the concurrent turning-on the switching elements 5 and 6 and the concurrent turning-off the switching elements 5 and 6 causes a ripple current as shown in FIG. 8(c). Thus, in the first mode, as shown in FIG. 8(b), when both the switching elements 5 and 6 are turned off, the energy generated in the phase coil 2 is supplied to the higher voltage line 3 of the power supply which is sometimes said to be xe2x80x98regenerationxe2x80x99, thereby decreasing the current rapidly.
In the second switching mode, as shown in FIG. 9(a) and FIG. 9(b), the first switching element 5 is alternately or cyclically turned on and off with the second switching element 6 remaining tuned-on. In this second mode, as can be seen from FIG. 9(c), the current which passes through the phase coil 2 becomes slightly rippled and when the first switching element 5 and the switching element 6 are being turned off and turned on, respectively, with the result that the current decreases in gradual manner, the driving force of and radially extending attraction in the motor decrease in a gradual manner. Thus, the resultant vibration and noise become relatively small.
In the third mode, as illustrated in FIGS. 10(a) and (b), the second switching element 6 is alternately or cyclically turned on and off with the first switching element 5 remaining tuned-on. In this third mode, as can be seen from FIG. 10(c), the current which passes through the phase coil 2 becomes slightly rippled and when the first switching element 5 and the switching element 6 are being turned on and turned off respectively, with the result that the current decreases in a gradual manner, the driving force of and radially extending attraction in the motor decrease in a gradual manner. Thus, like in the second mode, the resultant vibration and noise become relatively small.
In general, when a load is driven by the motor, the chopping energization control uses either the second mode or the third mode, while if regeneration is required when the load is braked by the motor, the chopping energization uses the first mode.
As well known, when currents pass through the respective switching elements 5 and 6, losses occur therein, thereby generating heat, respectively. If the resultant heat-generation becomes excess, the switching elements 5 and 6 are brought into thermal breakdown or heat breakage. To avoid such a drawback, a power down control method is employed which decreases the current passing through the phase coil 2 immediately when one of the switching elements reaches its permissible limit temperature, with the result that the thermal breakdown of the respective switching elements 5 and 6 can be avoided, but on the other hand, the output decreases due to the decrease of the current.
The losses occur when each of the switching elements 5 and 6 is turned on and is brought into switching action. In the second mode or the third mode of the chopping energization control, the first switching element 5 differs from the second switching element 6 in turned-on time duration and switching frequency, thereby differentiating the first switching element 5 from the second switching element 6 in amount of heat generation. Thus, a heat difference is generated between the first switching element 5 and the second switching element 6. The degree of such a difference depends on ratings of the electric motor and each of the switching elements 5 and 6. Sometimes the turning-on loss is larger than the switching loss and sometimes vice-versa. Assuming that if the turning-on loss is larger than the switching loss, in the second mode of the chopping energization mode, the loss of the switching element 6 becomes larger than the loss of the switching element 5, the temperature of the switching element 6 becomes higher than the temperature of the switching element 5. By contrast, in the third mode of the chopping energization control, the lass of the switching element 5 becomes larger than the loss of the switching element 6, thereby increasing the temperature of the switching element 5 is greater than the temperature of the switching element 6.
In the case where a temperature difference occurs between the first switching element 5 and the second switching element 6 and the temperature of either of the switching elements reaches the permissible limit temperature, power down control begins, even if the remaining switching element is still below its permissible limit temperature, thereby decreasing the output.
In view of the foregoing circumstances, a need arises to prolong the required time duration for each switching element to reach its permissible limit temperature.
It is, therefore, a principal object of the present invention to provide a chopping energization control device which satisfies the request noted above.
In order to attain the foregoing object, the present invention provides a chopping energization control device for use with an electric motor in such a manner that the chopping energization control device adjusts a current which passes through a phase coil of the electric motor, the chopping energization control device comprises:
(a) switching element driving signal generating means for generating a first driving signal and a second driving signal when the electric motor drives a load;
(b) a first switching element interposed between one end of the phase coil and a high potential line of a power supply;
(c) a second switching element interposed between the other end of the phase coil and a low potential voltage line of the power supply;
(d) the first switching element taking an on state under which the first switching element is turned on by receiving therein the first driving signal while the second driving signal is transmitted to the second switching element;
(e) the second switching element taking an on state under which the second switching element is turned on by receiving therein the first driving signal while the second driving signal is transmitted to the first switching element;
(f) the first switching element taking an on-and-off state under which the first switching element is turned on-and-off alternately by receiving therein the second driving signal while the second switching element takes the on state;
(g) the second switching element taking an on-and-off state under which the second switching element is turned on-and-off alternately by receiving therein the second driving signal while the first switching element takes the on state; and
(h) alternating means for repeating, in alternating cycles, the on-and-off state of the first switching element and the on-and-off state of the second switching element.